Magnetic signals produced from a living body include, for example, a cardiac magnetic field accompanying movement of the cardiac muscles of the heart and a brain magnetic field generated by neurotransmission of the brain. The magnetic field produced from such a living body is extremely weak, and the cardiac magnetic field is at 1 to 100 pico-Tesla whereas the cerebral magnetic field is at 10 to 1000 femto-Tesla.
On the other hand, there are magnetic noise components such as geomagnetism and a magnetic field from a power line, which are as large as several tens of micro-Tesla and larger than magnetic signals produced from the living body. Therefore, in order to detect a magnetic signal from a living body, it is important to remove a magnetic noise component.
As a method of measuring a weak magnetic signal produced from a living body, a method using a gradiometer has been proposed.
In the gradiometer, a measurement sensor for measuring a magnetic signal from a living body and a reference sensor for detecting an external magnetic field entering from the outside are used in pair. Of these sensors, the measurement sensor is placed close to the living body, and the reference sensor is disposed at a distance of about 5 cm to 10 cm from the measurement sensor.
Then, the measurement of a magnetic signal from the living body includes subtracting the magnetic field detected by the reference sensor as a noise component from the magnetic field detected by the measurement sensor.
In the measurement for an object, the object is placed in a magnetic shield chamber covered with a material having a large magnetic permeability such as permalloy, thereby preventing an external magnetic field from intruding.